I
NTEGRATED
C
IRCUITS
D
IVISION
CPC7594
R04 www.ixysic.com 13
The rising V
DD
lock out release threshold is internally
set to ensure all internal logic is properly biased and
functional before accepting external switch commands
from the inputs to control the switch states. For a
falling V
DD
event, the lock out threshold is set to
assure proper logic and switch behavior up to the
moment the switches are forced off and external
inputs are suppressed.
To facilitate hot plug insertion and power up control the
LATCH pin has an integrated weak pull up resistor to
the V
DD
power rail that will hold a non-driven LATCH
pin at a logic high state. This enables board designers
to use the CPC7594 with FPGAs and other devices
that provide high impedance outputs during power up
and configuration. The weak pull up allows a fan out of
up to 32 when the system’s LATCH control driver has
a logic low minimum sink capability of 4mA.
2.2.2 Hot Plug and Power Up Circuit Design
Considerations
There are six possible start up scenarios that can
occur during power up. They are:
1. All inputs defined at power up & LATCH = 0
2. All inputs defined at power up & LATCH = 1
3. All inputs defined at power up & LATCH = Z
4. All inputs not defined at power up & LATCH = 0
5. All inputs not defined at power up & LATCH = 1
6. All inputs not defined at power up & LATCH = Z
Under all of the start up situations listed above the
CPC7594 will hold all of it’s switches in the all-off state
during power up. When V
DD
requirements have been
satisfied the LCAS will complete it’s start up procedure
in one of three conditions.
For start up scenario 1 the CPC7594 will transition
from the all off state to the state defined by the inputs
when V
DD
is valid.
For start up scenarios 2, 3, 5, and 6 the CPC7594 will
power up in the all-off state and remain there until the
LATCH pin is pulled low. This allows for an indefinite
all off state for boards inserted into a powered system
but are not configured for service or boards that need
to wait for other devices to be configured first.
Start up scenario 4 will start up with all switches in the
all-off state but upon the acceptance of a valid V
DD
the
LCAS will revert to one of the legitimate states listed in
the truth tables and there after may randomly change
states based on input pin leakage currents and
loading. Because the LCAS state after power up can
not be predicted with this start up condition it should
never be utilized.
On designs that do not wish to individually control the
LATCH pins of multi-port cards it is possible to bus
many (or all) of the LATCH pins together to create a
single board level input enable control.
2.3 Switch Logic
2.3.1 Start-up
The CPC7594 uses smart logic to monitor the V
DD
supply. Any time the V
DD
is below an internally set
threshold, the smart logic places the control logic to
the all-off state. An internal pullup at the LATCH pin
locks the CPC7594 in the all-off state following
start-up until the LATCH pin is pulled down to a logic
low. Prior to the assertion of a logic low at the LATCH
pin, the switch control inputs must be properly
conditioned.
2.3.2 Switch Timing
The CPC7594 provides, when switching from the
ringing state to the talk state, the ability to control the
release timing of the ringing switches SW3 and SW4
relative to the state of the switches SW1 and SW2
using simple TTL logic-level inputs. The two available
techniques are referred to as make-before-break and
break-before-make operation. When the break switch
contacts of SW1 and SW2 are closed (made) before
the ringing switch contacts of SW3 and SW4 are
opened (broken), this is referred to as
make-before-break operation. Break-before-make
operation occurs when the ringing contacts of SW3
and SW4 are opened (broken) before the switch
contacts of SW1 and SW2 are closed (made). With
the CPC7594, make-before-break and
break-before-make operations can easily be
accomplished by applying the proper sequence of
logic-level inputs to the device.
